1. Field of the Invention
This invention pertains to the general field of voltage reference apparatus. In particular, it provides a new method for manufacturing a reference diode device with a near-zero temperature coefficient utilizing the unique characteristics of Schottky hot-carrier diodes.
2. Description of the Prior Art
The general purpose of reference devices is to provide a voltage reference that exhibits very little change in voltage over a broad range of operating temperatures. Even though the temperature dependence of diode characteristics tends to be linear for limited ranges, diode currents and terminal voltages are affected in nonlinear fashion by temperature variations over their overall normal range of use. Therefore, compensation for temperature dependence by combination of components with offsetting characteristics is difficult and limited to relatively small ranges.
In general, when a diode is operated in the forward direction, an increase in temperature results in a decrease in its forward threshold voltage. Within relatively small temperature changes, this relationship is inversely linear and can be usefully expressed by the following empirical equation: EQU V.sub.F (T.sub.1)-V.sub.F (T.sub.0)=-k(T.sub.1 -T.sub.0)
where V.sub.F is the diode terminal threshold voltage (forward voltage); T.sub.0 and T.sub.1 correspond to standard room temperature (25.degree. C.) and to the diode operating temperature, respectively; and k is the diode's temperature coefficient. This equation applies at constant diode current. See Schilling, Donald L. and Charles Belove, "Electronic Circuits, Discrete and Integrated," Second Edition, McGraw-Hill Book Company, New york, N. Y., pages 54-60. In practice, a typical temperature coefficient for a silicon diode is approximately 2 mV/.degree. C. Thus, for example, the typical silicon diode voltage of 0.7 V at 25.degree. C. and 1.0 mA of current decreases to 0.5 V if the temperature is raised to 125.degree. C. Similarly, a temperature decrease to -75.degree. C. would produce a diode voltage of about 0.9 V at 1 mA.
On the other hand, when a diode is used as a zener in reverse bias mode, the change of the zener (breakdown) voltage V.sub.Z as a result of temperature fluctuations within a relatively limited range is directly proportional to the temperature change. This direct relationship is also usually expressed in terms of a temperature coefficient k, as follows: EQU k=(.DELTA.V.sub.Z /V.sub.Z)/.DELTA. T
where V.sub.Z is the zener voltage and T is the diode's temperature. In practice, a typical temperature coefficient in the higher nominal zener range of operation is approximately 0.1 percent; that is, the zener voltage increases by 0.001 V.sub.Z for each 1.degree. C. of temperature rise. This coefficient value declines when using lower nominal zener voltages and actually becomes negative as field emission (tunnelling) becomes predominant over avalanche breakdown mechanisms.
Prior art techniques have exploited the positive temperature coefficient of reversed-biased p-n junctions combined in series with the negative temperature coefficient of forward-biased p-n junctions to produce a nearly constant net voltage across the combination. By properly selecting each component of the combination, a near-zero temperature coefficient of voltage change can be achieved over their linear range of operation by connecting them in series. To the extent that the temperature response of the individual components is nonlinear, though, such series configurations retain nonideal temperature coefficients characteristics.
U.S. Pat. No. 3,780,322 to Walters (1973) discloses an improved voltage reference device consisting of a series combination of forward and reverse biased p-n junctions with bulk semiconductive material. The resistance of the bulk material, which also changes with temperature, is used to compensate for the change of the voltage standard with temperature and is selected to produce a reference device with minimal voltage versus temperature deviation. Thus, this technique introduces a significant amount of undesirable dynamic impedance in the device.
Therefore, there still exists a need for an improved reference voltage device that exhibits a near zero temperature coefficient over a wide range of operating conditions. The method and apparatus described herein are directed at attaining this result by utilizing specific and unique characteristics of forward-biased Schottky hot carrier diodes, which afford a level of design flexibility not found in prior art series combinations of forward and reverse biased junctions.